1. Field of the Invention
The present invention relates to a fuel injection control device for an internal combustion engine.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2-169834 discloses an internal combustion engine having a spark plug and a fuel injector for injecting fuel directly into the cylinder, wherein, at a low engine load, a fuel injection is carried out only during a compression stroke, i.e., the fuel injector injects all of an amount of fuel to be injected during approximately the latter half of the compression stroke, and at a middle or high engine load, a divided fuel injection is carried out, i.e., the fuel injector injects a part of an amount of fuel to be injected during an intake stroke, and injects a remaining part of the amount of fuel to be injected during a compression stroke.
In this engine, a performance of the engine, for example, an engine torque, an amount of NC.sub.X produced, or an amount of smoke produced, is considerably changed when a control pattern of a fuel injection (e.g., a fuel injection only during an intake stroke, a fuel injection only during a compression stroke, a divided fuel injection during the intake and the compression stroke) is changed. Accordingly, a problem arises in that the full output of the engine cannot be obtained when a predetermined changing engine load (threshold engine load), at which the control pattern of the fuel injection is changed, is not properly determined.
For example, as shown in FIG. 6, an engine torque corresponding to the fuel injection only during the compression stroke is shown by a curve A, and an engine torque corresponding to the divided fuel injection is shown by a curve B. When the changing engine load is equal to Q.sub.X, i.e., an engine load corresponding to a point at which the curve A and the curve B cross, a full engine torque can be obtained. When the changing engine load is equal to Q.sub.Y, i.e., is smaller than Q.sub.X, however, a problem arises in that the full engine torque can not be obtained between Q.sub.Y and Q.sub.X because the engine torque corresponding to the curve B is smaller than the engine torque corresponding to the curve A between Q.sub.Y and Q.sub.X. Also, a similar problem arises when the changing engine load is equal to Q.sub.Z, which is larger than Q.sub.X.